Paper containing scalenohedral precipitated calcium carbonate (s-pcc)

ABSTRACT

The invention relates to paper that contains scalenohedral precipitated calcium carbonate (s-PCC) of a certain specification. The invention further relates to the user of a s-PCC having a certain specification as filler material for paper.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/EP2017/063168 file May 31,2017, which claims priority to EP 16172231.9 filed May 31, 2016, both ofwhich are incorporated by reference.

FIELD OF THE INVENTION

The invention relates to paper which contains scalenohedral precipitatedcalcium carbonate (s-PCC) having a certain specification. The inventionrelates further to the user of a s-PCC having a certain specification asfiller material for paper.

TECHNICAL BACKGROUND

Paper is a two-dimensional material which consists substantially offibres of plant origin and is formed by removing the moisture from afibre suspension on a sieve-like screen. The fibre fleece formed therebyis compressed and dried.

One of the main components of paper are cellulose fibres, which have alength in the range from a few millimetres to a few centimetres. Thecellulose is first largely exposed, that is to say separated from thehemicelluloses, resins and other vegetable components. The pulp obtainedthereby is mixed with water and defibred. The aqueous suspension isdeposited in a thin layer on fine-meshed screen and thickenedmechanically by moving the screen as it drips through.

When the paper has dried, its surface is impregnated (this process iscalled sizing).

The essential starter materials for paper can be divided into fourgroups.

-   a) Fibre materials (wood pulp, semi-chemical pulp, pulp, waste    paper, other fibres)-   b) Sizing and impregnating (animal glues, resins, paraffins, waxes)-   c) Filler materials (kaolin, talcum, gypsum, barium sulphate, chalk,    titanium white, etc.)-   d) Auxiliary materials (dyes, defoaming agents, dispersants,    retention agents, flocculants, wetting agents)

The present invention relates to the use of PCC as filler material. PCCis a synthetic industrial mineral which is manufactured from burned limeits raw material, limestone. Unlike other industrial materials, PCC is asynthetic product which can be shaped and modified to lend variousproperties to the paper that is to be produced. The physical form of thePCC can change considerably in the reactor. Variable factors include thereaction temperature, the speed with which the carbon dioxide gas isadded, and the speed of movement. These variables influence thegranularity and the shape of the PCC grain, its surface area and surfacechemistry, and the grain size distribution. While many advantages may begained from the ability to control the paper's properties with the aidof the PCC (greater brightness, impermeability to light and thicknessthan with ground calcium carbonate GCC), until now the possible use ofconventional PCC as filler material has been limited because it rendersthe fibres less stable.

In practice, conventionally manufactured s-PCC with an average grainsize D4.3 from 1.5 μm to about 5 μm is used as filler material inphotocopier paper among other things, although only up to a fillerpercentage of about 30%, because otherwise the tear resistance is toolow. It would therefore be desirable if it were possible to increase thedegree to which s-PCC can be used as filler without detracting from theproperties of the paper.

SUMMARY

The disadvantages described above can be overcome with the preparationof the paper according to the invention. For this purpose, the papercontains modified scalenohedral precipitated calcium carbonate (s-PCC).The s-PCC has a grain size distribution in which

${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100},$

preferably ≥60, particularly preferably ≥62 most particularly preferably≥65, and the average grain size D4.3 of the s-PCC is in the range from1.5 to 5.0 μm, particularly preferably 2.0 to 4.0 μm, particularly 2.9to 3.1 μm.

The invention is prompted by the realisation made experimentally that asthe D4.3 value of the s-PCC increases, the width of the grain sizedistribution increases as well, which has a detrimental effect on somepaper properties, such as tear resistance. It was demonstrated that anarrow grain size distribution is helpful in counteracting this.

The paper according to the invention is preferably a graphic paper.Graphic papers are papers that are used for printing, writing andcopying. As the need for graphic papers grows, a process technologydesigned specifically for these types of paper also become moreimportant.

The paper preferably has a grammage from 20 to 90 g/m², particularlypreferably 40 to 80 g/m², in particular from 50 to 60 g/m². Thus, thepaper may particularly be tissue paper (approx. 20 to 30 g/m²), Biblepaper (approx. 40 g/m²), newsprint paper or LWC paper (approx. 50 g/m²),notepaper or stationery paper (approx. 60 g/m²), typewriter paper(approx. 70 g/m²) or photocopier paper (approx. 80 g/m²). However, it isalso possible to fill “heavy” papers such as cartonboard with a grammagefrom 200 to 500 g/m² with this s-PCC according to the invention to goodeffect.

The degree to which the paper is filled with s-PCC of the statedspecification may be increased the percentage of the significantly moreexpensive pulp may be reduced correspondingly without impairing theessential paper properties. On the contrary, it has been found thatadding the s-PCC that is used according to the invention improvesimportant paper properties such as opacity, tensile strength andspecific volume.

The proportion of s-PCC as filler in the paper is in the range from 10%to 30% ash. The total quantity of the inorganic material contained in asample is called “ash”. When organic material is burned, substantiallyonly CO₂ and water vapour is generated, possibly also SO₂ or NH_(a).These gases dissipate, no residue remains. In contrast, the inorganiccomponents form salts or oxides, which typically do not even melt atnormal flame temperatures. The combustion residues, the ash, thuscontain all inorganic components of the sample. Incineration isunderstood to mean controlled combustion by heating to 575±25° C. untilno further weight loss is observed. Combustion must be carried outwithout atmospheric circulation and must not take place too intensely toprevent any fine fly ash from being transported away. The “ash” ofuncoated papers consists mostly of filler material, that of coatedpapers still contains the inorganic coating pigments. The quantity ofash is expressed as a percentage of the total weight of the (dried)paper mass.

Accordingly, a further aspect of the invention relates to the use ofscalenohedral precipitated calcium carbonate (s-PCC) with a grain sizedistribution for which the ratio

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

is resolved to:

${{\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100} \geq 59},$

and with an average grain size D 4.3 in the range from 1.5 to 5.0 μm asfiller material for paper. The ratio

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100\mspace{14mu} \left( {D\; {4.3/D}\; 90 \times 100} \right)$

is preferably ≥60, particularly preferably ≥62, most particularlypreferably ≥65. The average grain size D4.3 of the s-PCC is preferablyin the range from 2.0 to 4.0 μm, particularly 2.9 to 3.1 μm.

Thus, there was a need for s-PCC that has an average grain size D4.3 inthe range from 1.5 to 5.0 μm with a

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

ratio greater than or equal to 59. But until now an industrial processwhich can deliver such a material has not existed. However, this problemwas also solved according to the invention.

Many processes are known according to the related art in which PCC isformed in aqueous suspension from Ca(OH)₂ (“lime milk”) by the additionof CO₂. The CO₂ may be in liquid form or it may be introduced into thelime milk as a gas via a suitable ventilation system. The desired PCCmorphologies can be generated with the aid of additives or seed crystalsand with correspondingly adapted process management. In large-scaleproduction, a batch manufacturing mode is usually adopted.

Modifications to the PCC are described sufficiently in the literature.In the present context, the term “modification” is understood to referto the family of industrially manufactured crystals with definedmorphology, of which aragonite and calcite are particularly importantrepresentatives, and vaterite and ikaite are much less relevant. Thereare also special transition forms such as basic calcium carbonates oramorphous carbonates which can also be isolated. The person skilled inthe art in this industry will typically familiar with the limitconditions of the PCC system—some of which are highly complex—that mayinfluence the modification, from many successful and unsuccessfulexperiments. The fundamental control parameters, such as the starttemperature at the beginning of a typical batch cycle in conjunctionwith the concentration of the supplied lime milk and the CO₂concentration are specified in advance. Influences from the raw materialare eliminated in extensive test series, and if necessary renderedmanageable with various types of additives. It is also possible tomodify the degree of agglomeration of the PCC crystals by adjusting thereaction conditions.

However, until now there has been no systematic approach to theproduction of PCC with a defined grain size and a defined (narrow) grainsize distribution. The reason for this may lie in the complex interplayof a multitude of parameters. These include for example gassingparameters in conjunction with the CO₂ concentration, the geodeticallyeffective height of a reactor and the dissipative energy input.Furthermore, in a typical batch reaction, which is by far the mostfrequently used in industry, at any moment of the reaction importantprocess parameters can change in respect of a different characteristicin each case, for example the pH value of the suspension that is to begassed, the conductivity, the temperature, the ratio between freecalcium ions and bicarbonate ions, the density of the suspension and theviscosity of the suspension. The dynamics of all these changes are alsonot consistent at all times, some of the parameters, for example the pHvalue and conductivity do not change noticeably until close to the endof the batch cycle, but then they do so dramatically; meanwhile, otherparameters like the temperature rise, the change in density and thechange in viscosity manifest a practically linear change characteristic.This is further complicated by the fact that two main phases evidentlytake place during carboxylation, a preferred nucleation phase right atthe start of the reaction, followed by a particle growth phase whichtends to be preferred. As described in the recent literature, not eventhe particle growth takes place in linear manner, but instead via awhole series of intermediate states of immensely differing morphology.

For these reasons, information about which of the many phenomena aredecisive, controllable and adjustable for the characteristic variationof the average grain size (D4.3 value) and the width of the grain sizedistribution is at best very limited. Consequently, there are hardly anyindicators as what steps the person skilled in the art has to take withan existing PCC system in order to arrive at a product that has adefined grain size and also a defined grain size distribution.Therefore, until now it has not been possible to attain this objectiveconsistently.

U.S. Pat. No. 6,251,356 B1 suggests regulating the average grain size ina pressure reactor by controlling the order of the working pressure. Itis contented that the grain size ratio is narrower than withconventional process control. The process itself is technically verycomplicated.

EP 1 222 146 B1 relates to a two-stage, continuous process. In the firststage, a certain concentration of particles is generated. For this, thevolume flow rate of the lime milk can be changed with constant gas flowrate. In addition, influence can be exercised on the desired grain sizeby supplying a fine lime milk with increased reactivity.

According to Gernot Krammer et al. (Part. Part. Syst. Charact. 19 (2002)348-353), an increase in CO₂ concentration results in a reduction of theaverage grain size. An adverse influence of the CO₂ concentration on theaverage grain size is described by Bo Feng et al., Materials Science andEngineering A 445-446 (2007) 170-179 “Effect of various factors on theparticle size of calcium carbonate formed in a precipitation process”.

The concentration of the lime milk is another parameter that affects theaverage grain size (Kralj et Brecivic from Croatica Chimica Acta, 80(3-4) 467-484 (2007) “On Calcium Carbonates from fundamental research toapplication”). Higher solids contents in the lime milk usually lead tocoarser particles, lower solids contents should lead to finer particles.

It is also known that aragonitic crystals gradually become larger incontinuous operation of a PCC plant.

Pust (in a thesis entitled “Die Herstellung von gefälltëmCalciumcarbonat—PCC” [The production of precipitated calciumcarbonate—PCC] RVVTH Aachen, 1992) describes the influence of thequenching parameters in the production of quicklime on the size of thecrystals which are formed subsequently during carboxylation.

EP 1 712 597 A1 describes the effect of adding various additives such asZn-salts, Mg salts, and cationic and anionic dispersing agents on thegrain size distribution.

There was thus a continuing need for systematic solution approaches toenable the production of precipitated calcium carbonate of a definedgrain size and defined grain size distribution with a given PCC plant.The limitations of the prior art as described in the preceding textcould be solved with a recently developed method for producing s-PCC byintroducing carbon dioxide into lime milk in a PCC plant. The methodcomprises the following steps:

-   a) Capturing all parameters of the PCC plant which substantially    affect the specific molar energy input while the PCC plant is in    operation, wherein the specific molar energy input corresponds to    the energy input for the entire system that is needed to introduce a    mole of CO₂ to the carboxylation reaction in batch production mode    from the start of the reaction until a 90% degree of conversion is    reached;-   b) Determining the average grain size D4.3 depending on the specific    molar energy input;-   c) Determining the

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

ratio depending on at least one of the following parameters: CO₂concentration during the reaction, temperature of the lime milk, filllevel in the reactor of the PCC plant, and rotating speed of the gassingstirrer of the PCC plant; and

-   d) Introducing carbon dioxide into the lime milk while maintaining    the conditions of the requirements determined in steps b) and c).

There are various ways to describe the width or narrowness of the grainsize distribution. In the present context, it is characterized using theratio

${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100},$

which is often used in the field of particulates such as PCC. Here, D90means that 90% of the particles are smaller than the assigned value withvolume weighting.

The average grain size D4.3 is the arithmetic mean of a distributionacross all of the particles. A very narrow grain size distribution isobtained for example if the D4.3 is 3.1 and the associated D90 is 5.0μm. Then the

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

ratio yields the numeric value 62. The figures apply for the formationof the “primary particles” of the PCC process. Subsequent agglomerationsare ignored.

The size determination can be made with a laser diffraction particlesize analyser. All values are based on a dispersed product to preventagglomeration to the extent possible. In the present case, allmeasurements were taken using a particle size analyser either from thecompany Malvern (instrument name Malvern 3000) or Quantachrome(instrument name Cilas 1064 L). Both devices are very common in thepaper industry and consistently returned very similar values.

The newly developed method is based among other things on the findingthat the specific molar energy input is the decisive parameter forcontrolling the grain size. This value describes the sum of the specificenergy input of the overall system, which is required to input one moleof CO₂ in the decisive part of the reaction from about zero to 90% whenthe carboxylation reaction is proceeding in batch mode. The energy inputis to be measured without regard for its source. In a conventional PCCplant, particularly inputs from the ambient CO₂ concentration of thegas, the volumetric specific gas flow rate, the fill level of thereactor, the rotating speed of the frequency-controlled gassing turbineand/or the stirrer, the power output of an upstream fan will have to betaken into account.

Accordingly, in step a) of the method the individual influencing factorsof a PCC plant which deliver a significant contribution to the specificmolar energy input are captured. It has been found that the cumulativecontribution of all these influencing factors to the specific molarenergy input is in direct correlation with the target grain size.

Therefore, in step b) this correlation is calculated for the PCC plantin question. In general, it was found that the grain size decreases asthe specific molar energy input increases. For this purpose, preferablyin step b) a linear correlation is calculated between the average grainsize D4.3 and the specific molar energy input of the total system. Forthe determination of the correlation between the specific molar energyinput and the grain size at a specific PCC plant, in practice forexample a number of test settings are run with predefined specific molarenergy input and then the grain sizes are determined. The two values areplotted against each other and an associated linear function isdetermined by means of a graphical evaluation process. Now the requiredenergy input can be determined for a desired grain size with the aid ofthe function. Then, the influencing factors are adapted appropriately torepresent this energy input.

Surprisingly, it was then found that only the specific energy input ofthe substance delivery system represents the decisive parameter for thegrain size of the resulting crystals. It may be adjusted by anycombinations of the gassing parameters with the ambient CO₂concentration such that the target value for the desired grain size isformed. Thus, for the first time it is possible to set requirementsselectively for the grain size of the PCC crystals using the basicinformation about the characteristic values of the gassing apparatus.

In conventionally produced particle sizes, the ratio

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

lies in the range from about 2.8 μm and larger up to a maximum of 55 andis typically much smaller numerically. This is attributable to spinodalseparation processes during the carboxylation, which lead to theformation of continuously smaller zones with higher or loweroversaturation compared with the average value. As a consequence, notonly are new, smaller grains formed, but already existing crystals alsocontinue growing into larger crystals.

It was only during the course of the research for the present inventionthat it was demonstrated experimentally which influencing factors fromthe plethora of practically infinite possibilities are actuallysignificant for grain size distribution and are to be taken into accountwhen defining the respective desired value for the

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

ratio. The following parameters were identified: CO₂ concentrationduring the reaction, temperature of the lime milk, fill level of thereactor of the PCC plant, and rotating speed of the gassing stirrer ofthe PCC plant (step c) of the method). The four measures can be appliedsingly or in any combination and when observed coherently have theeffect of reducing the grain size distribution.

In particular it is preferable that in step c) the CO₂ concentration atthe start of the reaction corresponds to 0.5 to 0.8, particularly 0.6 to0.7 of the CO₂ concentration at the end of the reaction, and the CO₂concentration is increased progressively or incrementally. Thus, if afixed value for the CO₂ concentration of the source is known, as isalmost always the case (e.g., in power stations between 10 and 11%, inkilns for producing burnt lime about 22 to 26%, in biogas plants betweenabout 35 and 55%, or in synthetic gases about 98%), the startingconcentration is reduced to a value which is about 20 to 50%,particularly 30 to 40% smaller than the CO₂ concentration of the sourceand increased progressively or incrementally to the maximum possibleconcentration until the end of the reaction. This may be achieved mostsimply by diluting with air. Surprisingly, it was discovered that thisaction increased the ratio of the value for

${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100},$

particularly to 59 or more.

It is further preferable that in step c) a temperature is fixed at thestart of the reaction at which the PCC is precipitated in therespectively desired morphology, and that this temperature is keptconstant for the duration of the reaction or is lowered progressively orincrementally to 15° C., particularly 10° C. until the end of thereaction. Accordingly, the starting temperature is specified as thetemperature at which the desired morphology can form definitively. Inthe case of s-PCC, the starting temperature lies in the range from 25 to45° C. A starting temperature is also specified conventionally, but thetemperature is not controlled subsequently, and as a consequence thetemperature rises due to the exothermal nature of the reaction. Incontrast, according to the invention the temperature is kept constant orreduced incrementally or progressively to as low as 15° C. Surprisingly,it was found that this measure increases the value for

${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100},$

particularly to 59 or more.

It is further preferable that in step c) a fill level of the lime milkis equal to 50% to 80%, particularly 50% to 70% of the work volume ofthe reactor at the start of the reaction, and after a grain formationphase lime milk is added progressively or incrementally until the end ofthe reaction. Thus, at the start of the reaction the work volume of thereactor is only 50% to 80% filled with lime milk of the requiredstrength. Not before the end of the “grain formation phase”, inpractical operation for all morphologies this is the case after about 20minutes, more lime milk is added in measured quantities incrementally orprogressively, spread as evenly as possible over the entire T90 runtimeuntil the reactor has reached its nominal work volume. At the end of theT90 time, that is to say the significant reaction time in which 90% ofthe total conversion is complete, no more lime milk is added.Surprisingly, it was found that this measure increases the value for

${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100},$

particularly to 59 or more.

Finally, it is preferable in step c) that the rotating speed of thegassing stirrer is equal to 0.5 to 0.9, particularly 0.8 to 0.9 as fastat the start of the reaction as the speed at the end of the reaction,and the speed is increased progressively or incrementally to the finalspeed when 90% of the conversion reaction is reached. Accordingly, thereactor is started at the pre-calculated rotating speed, which wasselected such that a potential for increase of about 10 to 50% is stillpossible. After the grain formation phase is completed, but not laterthan the end of the T90 time, the rotating speed of the gassing stirreris increased progressively or incrementally by the remaining 10 to 50%until the end of the reaction. Surprisingly, it was found that thismeasure increases the value for

${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100},$

particularly to 59 or more. The s-PCC that may be obtained according tothe method has a particular combination of the grain size D4.3 and theratios

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100.$

The properties cannot be obtained with previously known PCC methods.

Further preferred variations of the invention will be evident from theclaims and from the following description.

BRIEF DESCRIPTION OF THE FIGURES

In the following text, the invention will be explained in greater detailwith reference to exemplary embodiments and associated drawings. Thefigures show:

FIG. 1 a schematic representation of a PCC plant;

FIG. 2 the results of batch carboxylations in the pilot reactor fors-PCC, wherein D4.3 is plotted as a function of the specific energyinput;

FIG. 3 the results of batch carboxylations in the pilot reactor fors-PCC, wherein D4.3 is plotted as a function of the specific energyinput;

FIG. 4 the particle size distribution of a s-PCC sample according to theinvention and of two comparison samples; and

FIGS. 5-10 comparative measurement results for paper samples withvarious content levels of s-PCC according to the invention andconventional s-PCC and GCC for the specific volume, stiffness, opacity,whiteness, tear length and thickness of each of the paper samples.

DETAILED DESCRIPTION

FIG. 1 is a—highly simplified—illustration of the basic construction ofa PCC plant 10 which may be used to carry out a PCC method: The methoddelivers s-PCC with a grain size distribution for which a numeric valueof ≥59 is obtained for the value

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

with an average grain size D4.3 in the range from 1.5 to 5.0 μm. The PCCplant 10 comprises a batch reactor 20, into which an aqueous suspensionof Ca(OH)₂ (also called “lime milk”) is introduced, and s-PCC is thenformed by the input of CO₂. The lime milk is fed in via a supply system30. Here, the CO₂ is mixed with the lime milk as a gas by means of asuitable ventilation system 40, which in this example comprises agassing stirrer 42. A further agitation mechanism 50 may be provided.The temperature in the reactor 20 is controllable. In this case, the PCCplant 10 contains sensor means—not shown in more detail—for monitoringthe fill level of the reactor 20 and capturing the temperature of thelime milk. Further sensor means may also be provided to enable a director indirect assessment to be made of a CO₂ concentration in the limemilk.

One of the bases for the invention is that the specific molar energyinput is the decisive parameter for controlling the grain size.Accordingly, all parameters of the PCC plant 10 which are ofsignificance for the specific molar energy input during operation, mustbe captured. In this context, the specific molar energy inputcorresponds to the energy input for the entire system that is needed tointroduce a mole of CO₂ to the carboxylation reaction in batchproduction mode from the start of the reaction until a 90% degree ofconversion is reached.

As is generally known, the process of calcium carbonate formation fromcalcium hydroxide and CO₂ is approximately linear in the main part ofthe reaction. After a value of about 90 to 95% of the total reaction,the pH and conductivity values fall sharply, and the CO₂ yield alsodecreases rapidly. Therefore, the average value for the yield of CO₂ inthe first 90% of the total reaction time is taken as the definingreaction time.

In order to measure the specific gassing rate, it is common to use thevalue “vvm”, which means: unit volume of gas per unit volume of reactorcontents per unit of time. In industrial practice and in typical reactorcontents of 10 m³, gassing rates of about 0.25 vvm to 5 vvm for exampleare customary, which after conversion mean that the reactor is gassedwith possible gassing rates from 150 Nm³ per hour up to about 3000 Nm³per hour. Smaller values are considered uneconomical, while largervalues are technically not possible because of the increasing risk thatthe gassing air in the reactor will merge. Merging means that when thepermissible gassing rate is exceeded the gas bubbles coalesce all atonce, and material transfer is no longer possible on a significantscale.

PCC plants are usually equipped with complex arrangements for supplyinggas to the reactor, which allow the CO₂ to be introduced as evenly aspossible over the full height and cross-section of the reactor with thelowest possible energy consumption. The energy consumption per hour ofthe overall system is derived substantially from the sum of the energyconsumption by the agitator elements and gassing turbines located in thereactor, which are motor-driven, together with any ventilator stationdelivering admission pressure. Indicators for energy consumptionrelative to the mass of PCC produced are typically in the range fromabout 60 kWh to 250 kWh per ton of PCC depending upon thecharacteristics of the gassing apparatus and the predetermined CO₂concentration. The plant operator generally has a very accurate idea ofthis energy consumption. For the sake of simplicity the total energyconsumed may be assumed to be the energy amount which behavesproportionally for the energy actually input. In the following section,the calculation of the specific molar energy input will be illustratedwith reference to an example.

Example Calculation

The following dataset is provided for carboxylation, wherein theobjective is to recover s-PCC:

Production of a reactive lime milk having 11% dry weight content ofcalcium hydroxide and with a density of 1,065 kg per m³, thus containing15.8 kmol calcium hydroxide in 10 m³. The viscosity of the lime milk isapproximately 50 mps.

The reactor is filled with 10 m³ of this lime milk.

Gassing is carried out at constant rate of 2,000 Nm³/h, corresponding toa vvm value of 3.33.

The CO₂ concentration is 26%.

The average utilisation factor of the CO₂ is 90%. The decisive time forcompletion of 90% carboxylation (T90 time) is 46 min.

The measured power requirement for a turbine is 130 kW.

The power consumption of an upstream fan is 40 kW.

The gas inlet temperature is adjusted to 40° C. by cooler.

The starting temperature in the reactor (lime milk) is 38° C. After thecarboxylation is 90% complete, the temperature in the reactor is 72° C.

Accordingly, in the 41 min of the reaction, 90% of the supplied limemilk was converted into s-PCC. This is equivalent to the formation of14.3 kmol or 1,430 kg s-PCC. A longer reaction time is needed for theremaining 10% of unconverted lime milk, because the specific conversionrate is known to decline at the end of the batch cycle.

The total energy consumption for the T90 time is 117 kWh.

During this period, 14.3 kmol CO₂ are input. The specific energy input(ε) per mole CO₂ is

$\frac{117\mspace{14mu} {kWh}}{14.3\mspace{14mu} {mol}\mspace{14mu} {CO}\; 2} = {8.2\mspace{14mu} {Wh}\text{/}{mol}\mspace{14mu} {CO}_{2}}$

The influence of the individual parameters on the specific molar energyinput is generally known, or can be determined easily by the personskilled in the art at a given PCC plant. Thus, for example the filllevel of the reactor may be plotted against the total power input (totalfrom fan, gassing unit, stirrer, etc.). The dependency of the gasutilisation on the ambient CO₂ concentration, the rotating speed of thegassing stirrer, the relative gas input, etc. can also be captured andevaluated.

FIGS. 2 and 3 show the results from test series which were conductedwith a pilot reactor and a technical reactor respectively. In each case,the characteristic average grain size D4.3 is plotted against thespecific energy input per mol CO₂ introduced in the decisive part of thebatch-reaction from 0 to 90%. In each case, the characteristic particlesize D4.3 was determined using a Mastersizer laser diffraction particlesize analyser produced by Malvern.

In one test series, scalenohedral crystals (s-PCC) with a grain sizeD4.3 in the range from approx. 1.1 μm to 3.0 μm are produced in thepilot reactor (FIG. 2) and the technical reactor (FIG. 3). In order toadapt the energy input, the CO₂ concentration, the fill level, the gasquantity and the rotating speed and admission pressure of the gassingturbine as well as other factors were varied individually or also incombination in the individual tests.

The following represents an exemplary dataset for the pilot reactor.

Example 1—Production of s-PCC in the Pilot Reactor

The following dataset was applied:

Lime milk 11.3 weight percentReactor fill level: 9 lSpeed of gassing turbine: 35 Hz

vvm: 0.5 (0.27 Nm³/h)

CO₂ concentration: 30%Reaction time T 90: 281 minGas utilisation rate: 81%CO₂ input in time T 90: 12.8 molesTotal energy input in the period T 90: 536 Wh sSpecific energy input per mole CO₂: 42 Wh per mol CO₂s-PCC was produced with a D4.3 of 2.38 μm.

It is evident that there is a direct relationship between the molarenergy input per mol input CO₂ and the resulting characteristic grainsize D4.3. The larger the amount of input energy applied to a mole ofCO₂, the smaller the crystals become, and vice versa. Surprisingly, itis therefore also possible to produce smaller particles with lowerconcentrations of CO₂ by combining corresponding parameters—for examplethose of gassing, the fill level and the rotating speed of the gassingdevices—provided that corresponding parameters are combined in suchmanner that the associated resulting specific energy input can berepresented.

The two following examples 2 and 3 show examples of s-PCC batches with avery large value for

${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100},$

which was achieved by selective variation of the test conditions.

Example 2—Production of s-PCC

The following dataset was applied:

Lime milk 11.3 weight percentReactor fill level: at the start of carboxylation: 220 l, after 20 min280 l, after 40 min 240 l until the end of the reactionSpeed of gassing turbine: at the start of carboxylation: 38 Hz, after 20min 40 Hz, after 40 min 45 Hz until the end of the reactionTemperature: 45° C., constant (heat dissipation via internal cooler)CO2 concentration: 45% (biogas), constantvvm: 1.5 per min, constantSpecific energy input per mole CO₂ immediately at the beginning of thereaction: 7 Wh per mol CO2s-PCC was produced with a characteristic D4.3 of 3.0 μm and a value for

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

greater than 62.

Example 3—Production of s-PCC

The following dataset was applied:

Lime milk 11.3 weight percentReactor fill level: at the start of carboxylation: 220 l, after 20 min280 l, after 40 min 240 l until the end of the reactionSpeed of gassing turbine: at the start of carboxylation: 38 Hz, after 20min 40 Hz, after 40 min 45 Hz until the end of the reactionTemperature: at the start of carboxylation 45° C., after 20 min 43° C.,after 40 min 41° C. until the end of the reaction (heat dissipation viainternal cooler)CO2 concentration: at the start of carboxylation 35%, after 40 min 45%vvm: 1.5 per min, constantSpecific energy input per mole CO₂ immediately at the beginning of thereaction: 8 Wh per mol CO2s-PCC was produced with a characteristic D4.3 of 2.9 μm and a value for

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

greater than 61.

Example 4—Production of Paper Samples

In general, filler materials for paper manufacturing are assessedaccording to a number of different criteria with regard to theirquantitative use. These criteria include processing-related featuressuch as sizing, retention, abrasiveness, mechanical strength, stiffness,and application-related features such as compressibility, porosity,roughness, surface energy, and finally optical properties such asopacity, whiteness, light scattering. Typically, a basic assessment iscarried out according to four criteria: thickness in μm, specific volumein g/m², opacity and stiffness. If these basic criteria for therespective paper are satisfied, most often the other properties can becorrected by readjustment. A filler material is particularly suitable ifit satisfies the basic criteria even when the filler content is high.

Paper samples with various filler materials and filler contents of 15%,20% and 24% ash in each case were produced as standard starter material.These papers were then referred to uniformly as “100% ash”. These paperswere also filled with further PCC until values of 135% ash and 175% ashfor the ash content resulted.

Samples A-1 to A-3: denote s-PCC obtained according to the methoddescribed above (=Inv. PCC) with a value for

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

of 59.3 and an average grain size D4.3 of 2.9 μm.

Samples B-1 to B-3: denote commercially available s-PCC (=HW PCC) with avalue for

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

of 55.75 and an average grain size D4.3 of 2.8 μm.

Samples C-1 to C-3: denote ground carbonates (GCC) (=HW GCC) with avalue for

$\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100$

of 53 and an average grain size of 1.8 μm. GCCs with D4.3 values largerthan about 1.8 μm are not used in paper production because of theirunacceptably high abrasiveness.

The particle size distribution in the samples was determined asdescribed earlier in this document, and is represented in FIG. 4. Thesize distribution in μm is plotted logarithmically along the x-axis, andthe y-axis shows the distribution as a percentage. As may be seen, thes-PCC according to the invention has a very narrow grain sizedistribution (Inv. PCC; solid line) compared with GCC (HW GCC; dottedline) and commercial PCC (HW PCC; dashed line).

The paper samples were produced in conventional sheet formers usual inthe industry, the experimental test conditions were prepared accordingto a standard dataset.

Then, the specific volume (FIG. 5), stiffness (FIG. 6), opacity (FIG.7), whiteness (FIG. 8), tearing length (FIG. 9) and thickness (FIG. 10)were recorded for paper samples having various levels of filling withs-PCC according to the invention and commercial s-PCC and GCC (fillinglevels 100% ash, 135% ash and 170% ash). The results of the analyses arepresented in FIGS. 5 to 10.

As may be seen, it was possible to increase the specific volume, theopacity, the tearing length, the whiteness and the stiffness. Asexpected, the thickness of the paper increased with no change ingrammage.

In general, it should be noted with respect to all measurements thatatmospheric humidity and temperature have a significant impact on themeasured values. For this reason, the measurements are always taken inair-conditioned rooms with a standard climate (23° C., 50% atmospherichumidity) fixed in accordance with ISO standards. The paper sample wasstored in the room for 24 hours before the measurement to enable it toacclimatise.

The degree of light impermeability of the paper (opacity) refers to itsability to block the passage of light. Paper is impermeable to lightwhen the incident light is scattered back or absorbed in the paper. Thegreater the scattering of the light, the more impermeable to light thepaper is. Light impermeability is a desirable quality which minimisesthe extent to which printed material can be seen through the back of thesheet. A sheet with 100% light impermeability prevents any light at allfrom passing through, and therewith also the printing on the sheetunless the printing ink penetrates the paper. In general, the lightimpermeability of paper decreases as the grammage gets lower. The degreeof whiteness and brightness of the filler material, its grain structureand size, its refractive index and the content of filler material arefactors which determine the light impermeability of paper. All importantproperties relating to paper-technology were maintained or improved byusing the PCC according to the invention despite an increase of almost50% in the degree of filling. The results were confirmed on apapermaking machine.

What is claimed is:
 1. Paper, containing scalenohedral precipitatedcalcium carbonate (s-PCC) having a grain size distribution for which${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100} \geq 59$and the average grain size D4.3 is in the range from 1.5 to 5.0 μm. 2.Paper according to claim 1, in which the paper has a grammage from 20 to90 g/m².
 3. Paper according to claim 1, in which the paper has agrammage from 200 to 500 g/m².
 4. Paper according to claim 1, in which${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100} \geq 60.$5. Paper according to claim 1, in which the s-PPC has an average grainsize D4.3 in the range from 2.0 to 4.0 μm.
 6. Paper according to claim1, in which a degree of filling of s-PCC in the paper is in the rangefrom 10% to 30% ash.
 7. Use of scalenohedral precipitated calciumcarbonate (s-PCC) having a grain size distribution for which${\frac{D\mspace{14mu} 4.3}{D\mspace{14mu} 90}{times}\mspace{14mu} 100} \geq 59$and with an average grain size D4.3 in the range from 1.5 to 5.0 μm asfiller material for paper.